A harmonica is a free reed musical wind instrument (also
known among other things as a mouth organ, french harp or
"Mississippi saxophone"), having multiple, variably-tuned brass
or bronze reeds, each secured at one end over an airway slot of like
dimension into which it can freely vibrate, thus repeatedly
interrupting an airstream to produce sound.

Unlike most free-reed instruments (e.g., reed organs, accordions
and melodicas), the mouth harmonica lacks a keyboard. Instead,
lips and tongue are used to select one or a few of the several holes
arranged usually linearly on a mouthpiece. Each hole communicates
with but one, two or a few reeds. Because a reed mounted above slot
is made to vibrate more easily by air from above, reeds accessed by a
mouthpiece hole often may be selected further by choice of breath
direction (blowing, drawing).

Some harmonicas also include a button-actuated slide that, when
depressed, further redirects the air.

The harmonica consists of a "comb" made of wood or plastic which
creates the holes into which a player blows or draws to make distinct
tones. The metallic blow and draw reedplates are screwed onto either side of the comb. Over the reedplates, there is a metal or plastic cover which projects the sound out of the open back. Chromatic harmonicas also have a button-activated slide.

The diatonic harmonica is most likely what you think of when you think
of a "harmonica." It has ten holes which offer the player 19 notes
(10 holes times a draw and a blow for each hole minus one repeated
note) in a three octave range. The standard diatonic harmonica is
designed to allow a player to play chords and melody in a single key.
Because they are only designed to be played in a single key at a time,
diatonic harmonicas are available in all keys. Here is a standard
diatonic harmonica's layout in the key of C:

Note that there is only one full major scale available on the
harmonica, between holes 4 and 7. The lower holes are designed around
the tonic (C major) and dominant (G major) chords, allowing a player
to play chords underneath a melody by blocking or unblocking the lower
holes with the tongue.

In addition to the 19 notes readily available on the harmonica,
players can play other notes by adjusting their embouchure and forcing
the reed to resonate at a different pitch. This technique is
typically called "bending" and allows to a player all the notes on the
scale as well as pitches in between. "Bending" creates the glissandos
characteristic of much blues and country harmonica playing. The
physics of bending are quite complex, but amount to this: a player can
bend a note down toward the pitch of the lower-tuned reed in that
hole. In other words, on holes 1 through 6, the draw notes can be
bent and on holes 7 through 10 the blow notes can be bent.

Howard Levy[?] developed another technique in the 1970s that allows players
to force a reed to vibrate faster, resulting in a higher pitch. This
technique is called overblowing or overdrawing and is much
less frequently used. For the few who master this technique, the
diatonic harmonica can function as a fully chromatic instrument.

The chromatic harmonica has a button-operated slide that allows the
player to change the pitch of any given hole. This means that each
hole has 4 pitches rather than 2. The slide typically shifts the
pitch of any given note by a half step. The note layout on a
chromatic is traditionally the same as the note layout on holes 4-7 of
the diatonic harmonica, and is repeated over its length. Chromatic
harmonicas are usually 12 or 16 holes long.

Because it is a fully chromatic instrument, the chromatic harmonica is
the instrument of choice in jazz and classical music. In traditional
harmonica bands, the chromatic harmonica plays the lead part.

The unrelated glass harmonica[?] is a musical instrument formed of a nested set of graduated glass cups mounted sideways on an axle and partially immersed in water, and played by touching the rotating cups with wetted fingers, causing them to vibrate.

There is an active harmonica community on the internet and in real
life, with conferences, cruises and everything. SPAH (Society for the
Preservation and Advancement of the Harmonica) is one society with a
particularly amusing acronym. [1] (http://members.aol.com/harmonica) A harmonica list-serv is hosted at
this web site (http://www.garply.com/harp-l/) with searchable archives.